Give a resource a URL

Mount a resource when its URL matters: a feed that readers subscribe to, a file that another system downloads, an export that users bookmark, or a well-known path such as /robots.txt. Without a mount, the URL of a resource starts with wicket/resource/ and contains a class name.

Mount a resource reference

Call mountResource in init() with the path and a resource reference. ResourceReference.of turns a supplier of the resource into a reference:

WicketApplication.java

@Override
protected void init() {
    super.init();
    mountResource("/exports/orders.csv",
        ResourceReference.of("orders.csv", OrdersCsvResource::new));
}

A request for /exports/orders.csv is now answered by the resource, and every link to the reference uses that URL. OrdersCsvResource is the resource from Generate a resource in code.

A mounted resource serves all users of the application. Keep no per-user state in it, and check in the resource whether the current user may see the data, because anyone can type the URL.

The path must not start with a segment that Wicket reserves for itself, such as wicket.

Mount a file from a package

A package resource can be mounted too, so that a fixed path answers requests that do not come from your pages. Links that Wicket renders to it add the version to the file name, as for any package resource; requests without the version still work.

mountResource("/robots.txt", new PackageResourceReference(WicketApplication.class, "robots.txt"));
mountResource("/favicon.ico", new PackageResourceReference(WicketApplication.class, "favicon.ico"));

The package resource guard still applies, so the file type must be one it serves; see Control which package resources are served.

Put parameters in the path

A segment written as ${name} is a placeholder. Wicket takes the value from the URL and passes it to the resource as a named parameter:

mountResource("/invoices/${number}", ResourceReference.of("invoice", InvoicePdfResource::new));

In the resource, read it from the attributes:

InvoicePdfResource.java

@Override
protected ResourceResponse newResourceResponse(Attributes attributes) {
    String number = attributes.getParameters().get("number").toString("");
    Invoice invoice = InvoiceService.get().findForCurrentUser(number);
    if (invoice == null) {
        return new ResourceResponse().setError(404);
    }

    byte[] pdf = InvoiceService.get().renderPdf(invoice);
    ResourceResponse response = new ResourceResponse();
    response.setContentType("application/pdf");
    response.setFileName("invoice-" + number + ".pdf");
    response.setWriteCallback(new WriteCallback() {
        @Override
        public void writeData(Attributes attributes) throws IOException {
            attributes.getResponse().write(pdf);
        }
    });
    return response;
}

Segments after the mount path become indexed parameters, and a query string becomes named parameters, as for pages: /invoices/2024-0042?download=true. Pass parameters to a page shows how to read both kinds.

Link to a mounted resource

Keep the reference in a constant, so links and the mount use the same instance, and pass parameters with PageParameters:

Resources.java

public final class Resources {
    public static final ResourceReference INVOICE =
        ResourceReference.of("invoice", InvoicePdfResource::new);

    private Resources() {
    }
}

WicketApplication.java

mountResource("/invoices/${number}", Resources.INVOICE);

InvoicePanel.java

add(new ResourceLink<Void>("pdf", Resources.INVOICE,
    new PageParameters().add("number", invoice.getNumber())));

The link renders /invoices/2024-0042. To get the URL as text, for example for an e-mail, use RequestCycle.get().urlFor(Resources.INVOICE, parameters), or Component#urlFor inside a component. Links to resource references are stateless, so they work on stateless pages.